It´s cold outside
Just as we thought it was about to be spring, the Siberian cold comes pouring. That will cost on the heating bill for the greenhouse if you have chosen to have the more delicate plants for overwintering in there. We are so happy that they survive that we underestimate the expense, but with 5 - 10 degrees night frost it is expensive in the budget.
Lower the floor
It is easier to move the heat around if it has been planned before setting up the greenhouse. For example, you can lower the floor so it is a bit below level, or you can insulate the floor in the same way you do in a house. If you do that, you have come a long way. The plinth must of course be insulated.
When you are still in the process of digging, you can make a one-meter-deep sand bed, where you lay floor heating with water which is then heated by solar panels. The bed is then heated with virtually no cost other than the set-up cost.
This is what the garden guru Anders Bech from Denmark has done. Anders keeps his greenhouse frost-free as long as the temperature outside does not go below minus 8 degrees.
Aside from last winter, which was somewhat harsh, that would pretty much mean a frost-free greenhouse all year round.
When the temperature outside has been between minus 8 to minus 16 degrees, the air in the greenhouse has been at a single minus degree, while the bed has always been frost-free.
It has required some digging. In the greenhouse, the room has been lowered one meter. The surrounding soil is held in place by the plinth, and the plinth is further insulated with polystyrene inside. Then the bed is filled with sand, and in the sand, the floor heating is laid layer by layer. The water is heated by the solar cell during the day, and the hot water gives off its heat to the sand at night.
It is so fortunate that with severe frosts, high pressure and sun often follow so that the solar cell produces its most in the severe frost. Alternatively, you can use an oil-fired boiler, but that is more expensive. It is also important to insulate the greenhouse itself. You can do that with bubble wrap, which many use when packing something that must not break.
The small solutions
If you do not have the courage for a big project, then there are other and easier options that can keep most greenhouses frost-free, as long as the cold does not bite too hard.
Bubble wrap is a super invention. It contains air and with special clamps, you can hang it up in the whole greenhouse, i.e., in the ceiling and on the walls. Since it is clear plastic, sunlight still gets in.
How cold or hot it is in the greenhouse will depend on the size of the room. Therefore, it is a really good idea to make the room smaller. Make some smaller rooms in the greenhouse so that the most delicate plants are in smaller rooms, where on a frosty night you can put out a few tealights as additional heat. Insulate around the plants with polystyrene but remember that insulation in itself does not make it frost-free in longer frost periods. Some kind of heat source is needed, and often a few tealights can handle the worst.
Horse manure heats
Other alternatives are petroleum lamps for greenhouses, which are quite cheap. The lamps can provide extra heat, and some burn for up to a week on a single filling but remember that fresh air must enter the house again, otherwise the plants die.
A slightly more expensive heating unit with a thermostat for gas is available from around 170 pounds. Gas emits quite a lot of water, so again ventilation is a must. However, gas is among the cheapest forms of heating. Right now, you cannot buy a gas heater with a thermostat anywhere on the Danish market, but Juliana has promised that they will look for options for themself. Maybe a wood stove or old tile stove could be an option. Coal and briquettes can stay red hot all night.
Instead of adding heat to the bed from the sun, horse manure can to some extent heat the soil.
Dig approximately half a meter into a bed. Fill the hole with 25 cm of fresh horse manure mixed with straw. Cover the straw with 5 cm of soil and give it a sprinkle of agricultural lime. Add another 10 cm of fertilizer twice, with a thin layer of soil and lime in between. The lime is intended to neutralize the acidic fertilizer.
Finish with a thin layer of straw and make some holes where you put your pots in. Now there will be good heat around the plant hole. Keep the bed warm by covering the ground with bubble wrap or a crop cover cloth.
Another option is to dig out the aisle, insulate it and put plants and tubers that do not need light down the hallway. Put on with some insulated tops that are strong enough to walk on.
Om Lars Lund
Danish horticulturist and journalist
Lars Lund has for many years engaged in the garden and greenhouse. Lars has published many books about greenhouses, and he has participated in many Danish horticultural TV shows. He is a walking garden encyclopaedia, and he has answers for most basic cultivation questions – also the more ambitious ones.
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